Beavers are two primarily nocturnal, semi-aquatic species of rodent, one native to North America (Castor canadensis) and one native to Eurasia (Castor fiber). They are very large with large, flat tails, and are the second-largest rodent in the world (after the capybara).

Known for building elaborate dams, canals, and lodges (homes), you'll generally find beavers in rivers, streams, marshes, lakes and ponds.

Beavers living along a river generally make burrows in the riverbank. Access to these burrows are by an underwater entrance. River beavers are known as bank beavers.

Those beavers that inhabit quiet streams, lakes and ponds usually build dams and a lodge. The dams are constructed of woven sticks, reeds, branches and saplings, caulked with mud. The lodges are dome-like in water, 6' (2 m) high or higher, up to 40' (12 m) wide. Their colonies create one or more dams to provide still, deep water to protect against predators, and to float food and building material.

The lodge has one or more entrances, all of them underwater. The living quarters are at the top of the dome in a hollow area. Beavers use wood chips on the floor to absorb excess moisture, and there is a vent to admit fresh air.

Beavers mate late January to late February and there are 1 to 8 kits (usually 4 or 5) born after gestation of 4 months.

When alarmed, beavers slap their tails on water loudly enough to be heard at a considerable distance.

Logs and twigs are peeled where bark is eaten. Felled trees and gnawed tree trunks; gnawings at considerable heights made when beavers stand on the surface of deep winter snow; successive gnawings, made when snow is at different levels, may produce a totem-pole effect.

Beavers use their preferred foods for construction materials: poplar, aspen, willow, birch and maple.

To fell a tree, the beaver gnaws around it, biting out chips in a deep goove. A beaver can cut down a 5 in (125 mm) thick tree in three minutes. After it has felled the tree, the branches are trimmed off and cut into convenient sizes, and are carried to the dam site. At this point, the beaver then eats the bark, turning the branches in its forefeet, much similar to the way that humans eat an ear of corn, or stores them underwater for winter use by poking the ends into the muddy bottom of the pond or stream.

The North American beaver population was once more than 60 million, but as of 1988 was 6-12 million. This population decline is due to extensive hunting for fur, for glands used as medicine and perfume, and because their harvesting of trees and flooding of waterways may interfere with other land uses.

Wikipedia has a couple of articles on Beavers.

An article on the North American Beaver.

An article on the American Beaver